In direct-injection internal combustion engines, reservoir injection systems (common rail systems) are increasingly being used, and the following demands are made of them: The injection pressure and the injection quantity should be definable independently of one another for every operating point of the direct-injection internal combustion engine, thus affording one additional degree of freedom for mixture formation. In addition, at the onset of injection the injection quantity should be as slight as possible, so that during the ignition delay between the onset of injection and the onset of combustion, not too much fuel will be introduced into the combustion chamber of a direct-injection internal combustion engine. In reservoir (common rail) injection systems with preinjection and main injection and having a modular design, the following components are used: Controlled injectors, which are screwed [in in] into the region of the cylinder head of the engine, pressure reservoir systems, and high-pressure pumps. The injectors communicate with the high-pressure reservoir via short lines and essentially include an injection nozzle and a triggering unit. The injected fuel quantity, for a given pressure, is proportional to the ON time of the actuating unit and is independent of the rpm of the engine and of the pump rpm. The requisite short switching times of the valve actuating units can be attained by designing them appropriately for triggering with high currents and voltages.
From German Patent Disclosure DE 198 35 494 A1, a unit fuel injector is known. It serves to deliver fuel to a combustion chamber of direct-injection internal combustion engines with a pump unit for building up an injection pressure and for injecting the fuel via an injection nozzle into the combustion chamber. A control unit with a control valve is also included; the control valve is embodied as an outward-opening A-valve. A valve actuating unit is provided for controlling the pressure buildup in the pump unit. To create a unit fuel injector with a control unit that has a simple design, is small in size, and especially has a short response time, this reference proposes that the valve actuating unit be embodied as a piezoelectric actuator.
From German Patent DE 37 28 817 C2, a fuel injection pump for an internal combustion engine is known. A control valve member comprises a valve shaft, which forms a guide sleeve and slides in a conduit, and a valve head connected to it and oriented toward the actuating device. The sealing face of the valve head is embodied to cooperate with the face of the control bore that forms the valve seat. The valve shaft, on its circumference, has a recess whose axial length extends from the discharge point of the fuel delivery line to the beginning of the sealing face on the valve head that cooperates with the valve seat. A face subjected to the pressure of the fuel delivery line is embodied in the recess and is equal in size to a face of the valve head that in the closed state of the control valve is exposed to the pressure of the fuel delivery line. As a result, in the closed state the valve is pressure-equalized, and a spring urging the control valve toward its open position is received in the guide sleeve.
It has been found that in injector designs that are used in reservoir (common rail) injection systems and that execute nozzle needle strokes of only a few tenths of a millimeter, throttle bores cannot be cleanly closed at such short strokes. As a result, unwanted leaks occur in the vertical motion of a control part in an injector housing and adversely affect the efficiency of an injector used in reservoir injection systems.
With the design proposed according to this invention, an injector for injecting fuel at high pressure into the combustion chambers of a direct-injection internal combustion engine, it is possible to use a 2/2-way valve instead of a 3/2-way valve. The incident leakage losses are significantly reduced by splitting the closing or relief throttle into two throttle elements. The reduction in leakage during the injection is achieved by closing the inlet of the second throttle element into the leaking oil line by means of a valve ring on a valve bolt. The valve ring can be embodied as a component surrounding a control part piston and including a plane end face and a conical end face. Both faces are acted upon via spring elements, which can be embodied as spiral springs. The spiral springs can be disposed in hollow chambers inside the injector housing. When the valve ring is moved into an annular chamber in the injector housing, the conical end face of the valve ring closes off the second throttle element from the leaking oil line. As a result, reduced leakage can be attained, which favorably affects the efficiency of the injector.
As the nozzle needle moves upward to enable the injection of the fuel into the combustion chambers of a direct-injection internal combustion engine during the injection phase, the second throttle element is closed off on the outlet side, so that the pressure from the high-pressure collection chamber (common rail), which prevails in the nozzle inlet, is maximally maintained. Thus the injection pressure course and the injection pressure level can be adhered to as calculated in advance, so that an injection pressure course corresponding to the course of combustion can be achieved.
The term xe2x80x9cinjection pressure coursexe2x80x9d means the varying fuel flow rate during one injection cycle, from the onset to the end of an injection. The course of injection determines the fuel mass pumped during the injection delay between the onset of injection and the onset of combustion. It affects the distribution of fuel in the combustion chamber and thus the air utilization upon combustion in the cylinder of a direct-injection internal combustion engine. The course of injection must rise slowly, so that as little fuel as possible will be injected during the ignition delay. With the onset of combustion, that is, after the development of a complete flame front, this fuel burns fiercely; a term also used is premixed combustion, which adversely affects noise production and NOx emissions. At the end of combustion, the course of injection must drop off sharply, to prevent poorly atomized fuel in the final phase from causing major emissions of hydrocarbons and soot build up, as well as increased fuel consumption in the direct-injection internal combustion engine.